In the related art, for example, an apparatus installed in a vehicle has been known in which another vehicle is detected by a millimeter wave radar device, vehicle density between the other vehicle which is within a predetermined distance from the vehicle and the other vehicle is calculated and, further, it is determined whether a traveling state of the vehicle is a cause of traffic congestion by using critical density in accordance with the speed of the vehicle to report the determination result to a driver (for example, refer to Patent document 1).